What causes those "Instant On" ESC fires?

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Nothing is forever. While electronics in general have become very reliable, especially in recent years with new technology, everything everywhere will fail at some time or another.

And flukes. A million units might have an incredible long life span, and a few dozen from the same batch will fail instantly, or at some random time.

I hate seeing the term "bulletproof" used on mechanical items. Everything fails at some time.

It is impossible to design an electronic device that will last forever, and the more you use elelctrical items the bigger the odds are that you will see failures.

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Also- your battery is fine. Just check the temps (like you are) and if temps are reasonable you have done everything you could to safeguard your system.

After that it's a crap shoot